Predation and survival among inadunate crinoids

The proposition is presented that the large thecae of many Paleozoic crinoids housed gonads, unlike modern crinoids that have the gonads on the arms or pinnules. Early in their history, inadunate crinoid gonads migrated into a voluminous anal sac, effectively separating them from other vital organs. Excision of the sac by predatory fishes and cephalopods would have been less traumatic than an attack on the theca, and the sac could be more readily regenerated. The pores and slits between plates on inadunate sacs are interpreted as gonopores. Some anal sacs may also have served as brood chambers. The traditional explanation of the sac as a special respiratory structure is discounted.